The present invention relates to medical diagnostic ultrasonic imaging, and in particular to imaging methods that increase the number of transmit focal zones while maintaining a high frame rate.
In ultrasonic imaging, beams of ultrasound energy are conventionally focused both during transmit and receive. Typically, one or two distinct transmit focal zones are transmitted for each scan direction. For each transmit focal zone a corresponding receive beam is formed. Generally, the receive beams are dynamically focused, while each transmit beam focuses the ultrasound energy at a distinct spatial position.
It is desired to increase the number of transmit focal zones in order to improve imaging resolution. However, increasing the number of transmit focal zones can result in a reduction in the frame rate, which is undesirable. This reduction in frame rate is particularly acute for single-beam transmit systems, due to the fact that only a segment of an image line is formed with each transmit focal zone, and many transmit focal zones are required to form a high resolution image.
One prior approach to this problem is to shape the transmit focus delay profile to extend the depth of field during transmit and receive. Piecewise focus and Axicon are examples of this approach. Both piecewise focus and Axicon techniques produce a relatively high side lobe level which is in many cases undesirable.
Hossack U.S. Pat. No. 5,608,690, assigned to the assignee of the present invention, describes a transmit beamformer with a frequency dependent focus. With this approach different frequencies of the transmitted beam are focused at different ranges. Hossack suggested in his paper entitled "Extended Focal Depth Imaging For Medical Ultrasound"(1996 International Ultrasonic Symposium, Nov. 5, 1996) that this frequency dependent focusing technique can be used in a multiple transmit zone configuration. As suggested in this paper, one zone may vary from 40 to 90 mm and the second zone may vary from 90 to 140 mm.
Pittaro U.S. Pat. No. 5,113,706 describes an ultrasound system with dynamic transmit focus. In one embodiment, three separate transmit events are used to create three separate transmit beams, all steered along the same scan direction but focused at different depths. This approach reduces the frame rate by a factor of three. At column 13, lines 35through 40, the Pittaro patent suggests that other embodiments provide for multiplexing coded transmission, for example using frequency multiplexing, so that the multiple wave fronts for a given steering position can be concurrent rather than successive.
Cole U.S. Pat. No. 5,856,955, also assigned to the assignee of the present invention, describes a transmit beamformer system that allows multiple transmit beams to be created from a single firing event. As shown in FIG. 1A, simultaneously transmitted transmit beams can differ both in scan direction and focal depth. The discussion starting at column 26, line 31 clarifies that the individual beams within a single transmit event can be separately programmed for frequency, delay, apodization, as well as other parameters.